The present invention is directed to improvements in molded golf ball construction and more particularly to improvements in molded golf ball core construction. The improved core is useful in producing balls having, among other things, superior sound and feel as well as enhanced playability characteristics. The present invention is also directed to the novel methods used in constructing the core and to golf balls produced utilizing the improved core construction.
Sound and feel are two qualities of golf balls which are typically judged subjectively. For the most part, however, soft sound (xe2x80x9cclickxe2x80x9d) and soft feel (i.e., low vibrations) are golf ball qualities desired by many golfers. If a soft feeling ball is mis-hit, the sting in the hands is not as great as if a harder feeling ball is hit improperly. A soft sounding ball has a soft low pitch when hit with any club, but particularly off a putter.
One way to achieve a soft sound and feel is to provide a softened layer between the core and the cover. The prior art teaches development of a three piece ball or a multi-layer cover. However, adding additional layers is costly and can sometimes lead to non-uniform layers.
The Molitor, et al. U.S. Pat. No. 4,650,193 patent describes a two-piece golf ball comprising a core and a cover. The core has a central portion of a cross-linked, hard, resilient material and a soft, deformable outer layer. The cover is a conventional cover. The soft, deformable outer layer of the core is integral with the core. It is formed by treating a slug of an elastomeric material with a cure altering agent, namely elemental powdered sulfur, so that a thin layer of sulfur coats the surface. The sulfur-coated slug is then cured in a molding cavity at temperatures greater than 290xc2x0 F., e.g., 325xc2x0 F., for 10-20 minutes, depending on core temperature.
According to the ""193 patent, sulfur on the surface of the slug penetrates a surface layer to a depth of about {fraction (1/16)} inch during curing. Wherever the core is exposed to sulfur, the conventional peroxide cure is altered, resulting in an amorphous soft outer layer. The portion of the core that is not touched by the sulfur cures normally and becomes relatively crystalline. The end result is a spherical core having a hardness gradient in its surface layers.
The present inventors seek to achieve somewhat of a similar effect using methods which do not require the addition of elemental sulfur to modify and soften the core surface such that the cure on the core surface is retarded. At the same time, the inventors seek to maintain the parameters of resilience and hardness of the finished ball at desired levels.
Resilience is determined by the coefficient of restitution (C.O.R.), the constant xe2x80x9cexe2x80x9d, which is the ratio of the relative velocity of two elastic spheres after direct impact to that before impact, or more generally, the ratio of the outgoing velocity to incoming velocity of a rebounding ball. As a result, the coefficient of restitution (i.e. xe2x80x9cexe2x80x9d) can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision. Hardness is determined as the deformation (i.e. Riehle compression) of the ball under a fixed load of 200 pounds applied across the ball""s diameter (i.e. the lower the compression value, the harder the material).
Resilience (C.O.R.), along with additional factors such as clubhead speed, angle of trajectory, and ball configuration (i.e. dimple pattern), generally determines the distance a ball will travel when hit. Since clubhead speed and the angle of trajectory are not factors easily controllable, particularly by golf ball manufacturers, the factors of concern among manufacturers are the coefficient of restitution (C.O.R.) and the surface configuration of the ball.
In this regard, the coefficient of restitution of a golf ball is generally measured by propelling a ball at a given speed against a hard surface and measuring the ball""s incoming and outgoing velocity electronically. The coefficient of restitution must be carefully controlled in all commercial golf balls in order for the ball to be within the specifications regulated by the United States Golfers Association (U.S.G.A.).
Along this line, the U.S.G.A. standards indicate that a xe2x80x9cregulationxe2x80x9d ball cannot have an initial velocity (i.e. the speed off the club) exceeding 255 feet per second (250 feet per second with a 2% tolerance). Since the coefficient of restitution of a ball is related to the ball""s initial velocity (i.e. as the C.O.R. of a ball is increased, the ball""s initial velocity will also increase), it is highly desirable to produce a ball having a sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of hardness (i.e. impact resistance) to produce enhanced durability.
The coefficient of restitution (C.O.R.) in solid core balls is a function of the composition of the molded core and of the cover. In balls containing a wound core (i.e. balls comprising a liquid. or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings.
An object of this invention is to develop a method for improving the sound and feel of a golf ball without adversely affecting the resilience or coefficient of restitution of the ball. The method does not require the addition of sulfur based chemicals to an uncured slug, in order to minimize the steps involved. In addition, the softer golf ball produces the playability characteristics desired by the more skilled golfer. It also enhances durability characteristics, as the outer skin is flexible and resists crack propagation.
These and other objects and features of the invention will be apparent from the following summary and description of the invention and from the claims.
Typically, cores or one piece balls are molded at very high temperatures (in the range of 295xc2x0 F. or higher) for very short periods of time (i.e. 10-20 minutes). The resulting cores have a hard surface with a softer inner core. This is due to the high temperature exotherm degrading and softening of the inner core. The inventors have found that by molding the cores at somewhat lower temperatures (i.e. lower than 295xc2x0 F.) for increased durations (i.e. times greater than 20 minutes), cores having softened surfaces are produced. The inventors have also learned that exposing the cores to water prior to the conventional curing steps likewise softens the core surface. The soft skin embodied on the core is durable and resists crack propagation, a useful feature for one piece balls.
The present inventors have developed novel methods for producing a golf ball having a spherical core which includes a central portion and surface or skin portion. The central portion is harder than the surface portion. The hardness of the central portion ranges from about 50 to 90 Shore C, and the hardness of the integral skin is in the range of about 30-70 Shore C. The skin comprises the radially outermost {fraction (1/32)} inch to xc2xc inch of the spherical core. A conventional cover (i.e. comprised of ionomers, urethane, balata, or other elastomer-based cover materials) is then molded over the spherical core.
In one embodiment of the invention, the outer surface of a slug is softer to a depth of up to xc2xc inch by controlling molding temperatures. The raw slug is placed in a mold cavity which is closed using 500 psi pressure. A steam set point is fixed, and steam is applied for a predetermined time period in the range of 25-30 minutes. A maximum mold temperature in excess of the steam set point temperature is achieved. A conventional cover is then molded over the core.
Another related but novel embodiment entails the process of immersing a slug in water prior to molding the core. Water is absorbed into the surface of the slug. The slug is subsequently molded by heating it to a sufficient molding temperature for a predetermined period of time to form a core. The softened skin is up to xc2xcxe2x80x3 in thickness. A cover is subsequently molded over the core to form a golf ball.
An advantage of the present invention is that the methods allow for usage of existing molding equipment to achieve the softened skin more economically. Extraneous chemicals need not be purchased. The step of coating the slug with elemental sulfur is eliminated. With respect to the exotherm method described herein, only the temperature and timing need be adjusted. Only water and an optional surfactant need to be added for the second embodiment.
The two piece construction used in preparing golf balls in accordance with the present invention is advantageous over three piece balls. There are fewer steps involved and the resulting soft skin is more uniform.
The methods disclosed herein can also be used in constructing one piece balls wherein the soft outer skin encompasses a harder inner core. The soft outer skin offers increased durability as the soft outer skin is flexible and resists crack propagation. Improved spin and control are also realized from the one piece construction.
These and other advantages of the invention will become apparent from the detailed description provided below.